…at a time. r=gw

As an initial step to reduce the reallocation churn a bit. This texture array grows up to 12 layers for any trivial page and goes up to 40+ on many sites (like the youtube front page) so it's far from enough but it's a start. Simple popups like Help > About Nightly don't need more than 4 layers, though.

Differential Revision: https://phabricator.services.mozilla.com/D67366

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/a781ce6dd0204621001b60c291601dd7f5666690